1. Field of Invention
Exemplary aspects of the present invention relate to an apparatus for image forming, and more particularly to an apparatus for image forming capable of effectively correcting a misregistration when an image is written.
2. Description of the Related Art
A related art image forming apparatus such as a copying machine and a printer, for example, is generally provided with an optical device to produce an optical beam and uses it to write image information on a photoconductor. Such a related art image forming apparatus may employ a plastic lens to provide the optical beam to meet recent trends of reducing costs and weights, for example. In addition, the related art image forming apparatus has increasingly penetrated its market and expanded the range of uses and, as a consequence, it needs to satisfy the market demands for a higher accuracy of an image magnification in a variable magnification operation.
The related art image forming apparatus is generally configured to modulate the optical beam based on image data, and to deflect the optical beam in a main scanning direction by a deflection mechanism (e.g., a polygon mirror) so as to make the optical beam scan a photoconductor surface through optical elements including an Fθ lens.
However, each of the related art image forming apparatuses may have a different main scanning magnification due to a variation in the optical device and/or properties of optical elements such as the Fθ lens. A defective operation, for example, a magnification error or a misregistration of a writing start position, may also be generated by a change in a refractive index or a shape of the plastic lens and a change in a scanning position on the photoconductor caused by a variation in an environmental temperature or a variation in a temperature of the apparatus therein. Thereby, an image with a high quality may not be provided. When a related art color image forming apparatus for forming a color image by superimposing a plurality of color images is used, the defective operation, including the magnification error or the misregistration of the writing start position, may be generated in a color basis. Thereby, the main scanning magnification or writing start position for each color may need to be corrected.
One example has attempted to correct a registration of an image in units of 1 pixel or less by using a phase variable function. This phase variable function, for example, varies a phase of writing clock signals in units of 1/n of a writing clock cycle, where n is an integer of 2 or greater, at a position or a plurality of positions in the main scanning direction.
According to the above example, the image forming apparatus and a light scanning device of the related arts correct the misregistration of the main scanning position by having the phase variable control function to control a variation of the phase of writing clock signals as a correction mechanism. The phase of the writing clock signals may be varied in units of 1/n of the writing clock cycle, where n is an integer of 2 or greater, at the one position or the plurality of positions in the main scanning direction by the phase variable function so that the misregistration of the main scanning position is corrected.
Referring to FIG. 1A, the misregistration of the main scanning position may be corrected by varying the writing clock phase when n is 4, i.e., 1/n=¼, for example, based on a reference writing clock signal.
Referring to FIG. 1B, when the writing start position for the main scanning is corrected, the phase of writing clock signals generated between two detection signals A and B is varied.
Another example has attempted to correct the main scanning magnification by correcting the writing clock based on a comparison with a predetermined reference count number. According to this example, a plurality of optical beam detection mechanisms to detect the optical beam on a main scanning line are employed so as to measure an interval between the optical beam detected by one of the plurality of optical beam detection mechanisms and the optical beam detected by another optical detection mechanism by using a predetermined clock number.
When the main scanning magnification is corrected by the example stated above but under a situation of FIG. 1B, the count number of the writing clock signals may vary an amount which is a correction amount generated by the phase variable function, and a frequency of the writing clock signals may be varied at the same time. Thereby, the main scanning magnification may be affected.
Referring to a flowchart of FIG. 2, one example of a related art correction procedure is explained, for a correction of the main scanning magnification in a related art image forming apparatus. In a step 1201, a scanning time (e.g., a count number) NC between optical beam detection mechanisms is counted. When a clock counter in a writing clock generator is cleared by a detection signal of a synchronous detection plate, measurement clock signals are counted by the clock counter. A count number of the measurement clock signals counted by the clock counter is output to a control unit by a detection signal of a rear-end synchronous detection plate.
In a step 1202, the count number NC and a reference count number NR are compared with each other by the control unit. The reference count number NR is a predetermined number. In a step 1203, a writing clock frequency is corrected by computing f′=f×NR/NC, where f′ is a writing clock frequency after a correction is made, and f is the writing clock frequency before the correction is made.
According to the above procedure, the count number and the writing clock number frequency which are at least substantially the same as those between the optical beam detection mechanisms are computed. The control unit outputs the computed writing clock frequency to a writing clock generator so as to correct the main scanning magnification.
However, the measured count number may be varied when a phase of the writing clock signals is varied by a phase variation function because the writing clock signal is used to measure the scanning time.